Here the normalization factor \({\cal N}\equiv \frac{1}{\sqrt{2p_{0}V}}\)  associated with \(p_{0}\)  and space volume \(V\)  is now fixed so that the
massless photon energy \(\omega \)  in the electromagnetic wave can be given by
\(\omega =p_{0}=|\vec{p}|\)  [1]}. Note that \(\phi ^{a(m=0)}(x^{\mu })\equiv A^{a}(x^{\mu })\)  is the four-vector potential in the
elecromagnetism for the massless photon with spin one.
